* {
  box-sizing: border-box; /* padding and border are now included inside the declared width/height instead of adding to it */
}

body {
  font-family: Arial, sans-serif;
  background-color: #f0f2f5;
  margin: 0;
  padding: 20px;
}

.container {
  max-width: 500px;
  margin: 0 auto;
  background: #ffffff;
  padding: 24px;
  border-radius: 10px;
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.1); /* rgba() sets color with an added alpha value for transparency — black at 10% opacity here */
}

h1 {
  text-align: center;
  color: #2c3e50;
}

form {
  display: flex;
  flex-direction: column;
}

label {
  margin-top: 14px;
  margin-bottom: 6px;
  font-weight: bold;
  color: #34495e;
}

input,
select,
textarea {
  padding: 10px;
  border: 2px solid #ccc;
  border-radius: 4px;
  font-size: 14px;
}

input:first-of-type { /* :first-of-type is a structural pseudo-class matching only the first <input> among its siblings */
  border-radius: 20px;
}

input:focus,
textarea:focus {
  border-color: #3498db;
  outline: none;
}

input:invalid,
select:invalid,
textarea:invalid { /* :invalid is a native pseudo-class that auto-matches any field currently failing its own built-in validation — no JS needed */
  border-color: red;
}

input:valid,
select:valid,
textarea:valid { /* :valid is the inverse — matches fields currently passing validation */
  border-color: green;
}

textarea {
  min-height: 90px;
  resize: vertical; /* lets the user drag-resize the textarea vertically only; horizontal resize is disabled */
}

.radio-group {
  border: 1px solid #ccc;
  border-radius: 6px;
  margin-top: 14px;
  padding: 10px 14px;
}

.radio-group legend {
  font-weight: bold;
  color: #34495e;
}

.radio-group label {
  margin-left: 6px;
  margin-right: 20px;
  font-weight: normal;
  display: inline;
}

.radio-group input[type="radio"] { /* attribute selector — targets only inputs whose type attribute is exactly "radio" */
  width: 16px;
  height: 16px;
}

.radio-group input[type="radio"]:checked {
  border-color: #3498db;
  background-color: #3498db;
  box-shadow: 0 0 5px 1px rgba(52, 152, 219, 0.8);
}

.radio-group input[type="radio"]:checked + label { /* the + is an adjacent-sibling combinator — matches an element only when it comes immediately after the matched one in the markup */
  color: #3498db;
}

button {
  margin-top: 20px;
  padding: 12px;
  background-color: #3498db;
  color: #ffffff;
  border: none;
  border-radius: 4px;
  font-size: 16px;
  cursor: pointer;
}

button:hover {
  background-color: #2980b9;
}